1. Field of the Invention
The present invention is directed to an improved method of generating visible light and to an improved bulb and lamp for providing such light.
2. Related Art
U.S. Pat. Nos. 5,404,076, and 5,606,220, and PCT Publication No. WO 92/08240, which are incorporated herein by reference, disclose lamps for providing visible light which utilize sulfur and selenium based fills. U.S. application Ser. No. 08/324,149, filed Oct. 17, 1994 (U.S. Pat. No. 5,661,365), also incorporated herein by reference, discloses similar lamps for providing visible light which utilize a tellurium based fill.
These sulfur, selenium and tellurium lamps of the prior art provide light having a good color rendering index with high efficacy. Additionally the electrodeless versions of these lamps have a very long lifetime.
Most practical embodiments of sulfur, selenium, and tellurium lamps have required bulb rotation in order to operate properly. This is disclosed in PCT Publication No. WO 94/08439, where it is noted that in the absence of bulb rotation, an isolated or filamentary discharge results, which does not substantially fill the inside of the bulb.
The requirement of rotation which was generally present in the prior art lamps introduced certain complications. Thus, the bulb is rotated by a motor, which has the potential for failure, and which may be a limiting factor on the lifetime of the lamp. Furthermore, additional components are necessary, thereby making the lamp more complex and requiring the stocking of more spare parts. It therefore would be desirable to provide a lamp affording the advantages of the prior sulfur, selenium and tellurium lamps, but which does not require rotation.
PCT Publication No. WO 95/28069, a Dewar lamp was disclosed for purportedly obviating rotation. However, a problem with such Dewar configuration is that it is complicated in that it utilizes peripheral and central plated electrodes on the bulb, and the central electrode is prone to overheating.
The present invention provides a method of generating visible light, and a bulb and lamp for use in such method which eliminates or reduces the need for bulb rotation.
The invention affords increased design flexibility in providing lamp bulbs of smaller dimensions and/or utilizing sulfur, selenium or tellurium fills having lower density of active substances than in the prior art, which are still capable of providing a primarily visible light output. This, for example, facilitates the provision of low power lamps, which may lend themselves to the use of smaller bulbs. This feature of the invention may be used in combination with other features, or independently. For example, a smaller bulb may be provided either which doesn""t rotate, or which does rotate.
In accordance with a first aspect of the present invention, a method is provided utilizing a lamp fill which upon excitation, contains at least one substance selected from the group of sulfur and selenium; the lamp fill is excited to cause said sulfur or selenium to produce radiation which includes a substantial spectral power component in the ultraviolet region of the spectrum, and a spectral power component in the visible region of the spectrum, the radiation is reflected a multiplicity of times through the fill in a contained space, thereby converting part of the radiation which is in the ultraviolet region to radiation which is in the visible region of the spectrum, which visible radiation is greater than it would have been if reflecting had occurred in the absence of the conversion. Finally, the visible radiation is emitted from the contained space.
In accordance with a further aspect of the invention, the fill is excited to cause the sulfur or selenium to produce a spectral power component in the ultraviolet and a spectral power component in the visible region, wherein the multiple reflections result in a reduced ultraviolet spectral component having a magnitude of at least 50% less than the original component.
In PCT Publication No. WO 93/21655 sulfur and selenium lamps are disclosed in which light is reflected back into the bulb to lower the color temperature of the emitted light or to make it more closely resemble the radiation of a black body. Unlike in the present invention, in the prior art system it is radiation having an essentially visible (and higher) spectral output which is reflected to produce another visible spectral output having more spectral power in the red region. In distinction to the prior art, in the present invention, the radiation which is reflected has substantial spectral power component in the ultraviolet region (i.e., at least 10% of the total of the ultraviolet and visible spectral power), of which some is converted to the visible region. It is this conversion of ultraviolet to visible radiation in the present invention by multiple reflections which allows a small bulb to replace a larger one and/or the use of a lower density of active material which allows stable operation to be achieved without rotating the bulb.
Inasmuch as the method of the invention involves multiple reflections of light through the fill, and finally to the outside, it was contemplated that a bulb be used which has a reflector layer around the quartz, except for an aperture through which the light exits. Such xe2x80x9caperture lampsxe2x80x9d are known in the prior art, and an example is shown in U.S. Pat. No. Re 34,492 to Roberts.
The Roberts patent discloses an electrodeless spherical envelope having a reflective coating thereon, except for an aperture which is in registry with a light guide. However, it has been found that the Roberts structure is not suitable for practicing the method of the present invention as it would be employed in normal commercial use. This is because of its use of a coating on the lamp envelope. When the bulb heats up during use, the different thermal indices of expansion of the quartz envelope and the coating cause the coating to crack. Thus, the lifetime of the bulb is quite limited. Also, a coating is not normally thick enough to provide the degree of reflectivity which is required to provide adequate wavelength conversion from ultraviolet to visible.
In accordance with an aspect of the present invention, these problems are solved by utilizing a diffuse, reflecting ceramic covering for the bulb which contacts at least one location of the envelope, and which does not crack due to differential thermal expansion. In a first embodiment, the covering comprises a jacket which unlike a coating, is non-adherent to the bulb. The lack of adherence accommodates the thermal expansion of bulb and jacket without causing cracking of the jacket. Also, the jacket is made thick enough to provide high enough reflectivity to accomplish the desired wavelength conversion. In a second embodiment, the reflective bulb covering is made of the same material as the bulb, so that there is no problem with differential thermal expansion. In this embodiment, the covering may additionally be in the form of a non-adherent jacket. In a further embodiment, a diffusely reflecting powder is disposed between a jacket and the bulb.